You're never charged extra for using more data. What happens instead depends on whether your plan comes with a set amount of data or is unlimited.
If your plan has a set amount of data
When you've used it, your data stops until your cycle renews. It doesn't slow down, it stops. Calls and texts carry on working normally, so you can still use your phone, you just need wifi or a top up for anything that needs data.
If your plan is unlimited
You get full speed up to a point in each cycle, and after that your speed is reduced for the rest of it. You're never cut off and there's no extra charge. Where that point sits depends on which unlimited plan you're on, so check your plan for the exact amount.
What reduced speed feels like
Messaging, email and maps generally still work. Video, large downloads, video calls and online gaming are what suffer.
Video quality
Video streams at up to 480p on every Flex Mobile plan, all the time, not only after your high speed allowance runs out. That's a deliberate network setting, not a sign that you've been slowed down.
Network congestion is different
Even while you still have high speed data, speeds can drop for a while when the network is busy. That's deprioritisation. It's short lived and it clears as demand falls.
When your speed comes back
At the start of your next billing cycle.
If you need more before then
Both options are in your Flex Mobile account. You can buy a data top up, which adds high speed data straight away, or you can change to a larger plan. If you're running out most months, changing plan usually costs less than repeating top ups.